patient is usually unconscious for some hours, and very often the mechanism of injury is
conjectural. It is agreed, however, that in traction injuries the essential factor is forcible
separation of the head and shoulder, the position of the upper limb at the moment of impact
determining the particular roots of the plexus which will receive the brunt of injury.
The brachial plexus is protected by the scalene muscles and by a strong layer of deep
cervical fascia. Violence must be severe enough to tear the fascia, and rupture the scalene
muscles, or to avulse the tubercles of the transverse processes from which the muscles arise,
before stretching of the nerve roots can occur. \Vhen the shoulder is forcibly depressed with the arm by the side the greatest stress
falls on the upper roots, and it is impossible to put the lower roots under tension (Fig. 1).
The violence may even cause downward dislocation of the first rib, but the lower roots still
escape injury if the limb is by the side. Some additional factor must therefore operate when
the lower roots are damaged irreparably. It can be shown by experiment on the cadaver that tension is exerted on all roots of the plexus when the abducted limb is forced behind
the trunk and the head is thrust towards the opposite side (Fig. 2). The tension on each root varies with the position of the limb: elevation increases tension on the lower roots; adduction
increases tension on the upper roots. These observations are in accord with clinical findings,
for lesions of the whole plexus may be divided into three types: 1) lesions with permanent
paralysis of all muscles of the limb; 2) lesions in which there is early return of voluntary
power in muscles innervated by the lower roots, and no recovery in muscles innervated by
the upper roots; 3) lesions in which there is late return of voluntary power in muscles
innervated by the upper roots, and no recovery in muscles innervated by the lower roots.
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